1. Field of the Invention
The present invention relates to an image forming apparatus for fixing an unfixed image formed on a transfer medium, and a control method therefor.
2. Related Background Art
An image forming apparatus for forming an image by an electrophotographic process is provided with a charging unit for uniformly charging a photosensitive surface of a photosensitive drum. It is also provided with a latent image forming unit for forming an electrostatic latent image corresponding to image information on thus charged photosensitive surface, a developing unit for developing such electrostatic latent image, a transfer unit for transferring the developed latent image onto a recording material, and a fixing apparatus for fixing thus transferred image onto the recording material.
Such fixing apparatus is equipped with a heating member which generates heat by a current supply. Such fixing apparatus includes a heat roller type, utilizing a halogen heater as a heating member, and a film heating type, utilizing a ceramic heater as a heating member. In the fixing apparatus of the heat roller type, a recording medium is introduced in and conveyed through a fixing nip portion formed by a fixing roller maintained at a predetermined temperature by heating with a halogen heater, and an elastic pressure roller maintained in pressed contact therewith, whereby an unfixed toner image on the recording medium is heat fixed by the heat of the fixing roller. In such type, since the fixing roller has a large heat capacity, a long time is required for reaching a predetermined temperature for executing a fixing operation, thereby resulting in a waiting time. Also in order to elevate the temperature within a short time, it is necessary to pre-heat the fixing apparatus by a current supply thereto during a stand-by state of the image forming apparatus, whereby the electric power consumption may become higher.
In consideration of these points, a fixing apparatus of film heating type is practiced commercially (such type being hereinafter represented as on-demand fixing method). In a fixing apparatus of film heating type, a recording material is contacted with a heating member, supported on a support member, across a thin heat-resistant film material, whereby the heat of the heating member is transmitted to the recording material through the film material. In this type, there can be employed so-called ceramic heater, basically constituted of a substrate of a low heat capacity enabling a fast temperature elevation, such as an insulating ceramic substrate of a high thermal conductivity, and a heat-generating resistor layer provided on the surface of such substrate and generating heat by a current supply. Also there can be employed a thin film material of a low heat capacity, whereby the temperature of the fixing apparatus can be elevated within a short time. Thus the fixing apparatus need not be energized during the stand-by state and it is rendered possible, when a recording material to be heated is introduced, to heat the fixing apparatus to a predetermined temperature before the recording material reaches the fixing nip portion. In this manner it is made possible to shorten the waiting time and to save the electric power consumption, and to suppress a temperature elevation in the main body of the image forming apparatus.
Such fixing apparatus is equipped with a temperature control apparatus as shown in FIG. 10. A heating member 1001 is connected, through a switching element 1002 such as a triac, to an AC power source 1007 such as a commercial power source, which supplies an electric power. Also there is provided a temperature detecting element 1003, such as a thermistor, which detects the temperature of the fixing apparatus. Information on the detected temperature is subjected to an analog-to-digital conversion by an A/D converter 1004, and supplied to a personal computer 1005. Based on the entered temperature information, the computer 1005 supplies a control circuit 1006 with control information so as to reach a predetermined temperature, and the control circuit 1006 controls an on/off (current supply/current non-supply) operation of the switching element 1002. In this manner the switching element 1002 controls a duty ratio of the AC power supply to the heating member 1001.
Such on/off control is executed by a wave number control or a phase control of the AC power source. The wave number control or the phase control is executed by a triggering based on a signal which includes a point where the entered AC power supply is switched from positive to negative or from negative to positive and which indicates that a magnitude of the power supply voltage has reached a certain threshold value or less (such signal being hereinafter represented as zero-cross signal). More specifically, a temperature control by a phase control is executed by changing a phase angle of the AC current based on the temperature information detected by the temperature detecting element (for example by controlling a switch timing of the triac). Such phase angle control may be executed by a method of executing a temperature detection and determining a phase angle accordingly, or by a method of executing a temperature detection at a constant interval and adopting a predetermined output pattern accordingly. Such output pattern is executed with a combination of a predetermined fixed wave number and a phase angle enabling an optimum temperature control as a function of the temperature.
Also a wave number control method controls an on/off state for every half wave of the power supply waveform, as shown in FIG. 11. A predetermined electric power is supplied to the heating member 1001 by a current supply for example in solid-lined portions. In such wave number control, in order to achieve an extract temperature control of the fixing apparatus, the control is executed by setting an on/off pattern, taking plural half-wave portions as a block. More specifically, the control is executed utilizing a control table, which sets an on/off pattern for controlling the power supply amount to the heating member per unit time, by taking plural half-waves as a control block and by selecting an on/off ratio (duty ratio) in the unit of a half-wave.
On the other hand, the non-demand fixing method is known to be associated with a following drawback. In the on-demand fixing method, because of a low heat capacity, the precision of the temperature control is improved by frequency changing the electric power supply amount. Therefore, the electric power supply amount changes more frequently than in the heat roller method. For example, in the heat roller method, the temperature can be maintained within a predetermined range by an electric power change in every 5 seconds, because of a large heat capacity. On the other hand, in the on-demand fixing method, the temperature cannot be maintained within a predetermined range unless the electric power is changed several times within a second. Such fluctuation in the electric power consumption (current consumption) induces a fluctuation in the power supply voltage. Particularly in case of a power source of a high line impedance (for example in case of a long distance from a transformer in a power supply line and a power supply line of a high resistance), the power supply voltage fluctuates frequently and significantly, thus causing a flickering of illumination or a television image (such phenomenon being hereinafter called flickering). In the wave number control, for example in case of controlling the duty ratio by a unit of 5%, the control is executed for example by taking 20 half-waves as a group and turning on several half-waves in a former half and turning off several half-waves in a latter half for achieving an electric power control in 10 levels, and, in such case, a frequency of current change becomes as low as 5 Hz whereby the flickering becomes easily noticed by human eyes.
Also in an image forming apparatus of a higher process speed requiring a larger electric power, a maximum electric power has to be increased by reducing the resistance of the heater, but the flickering phenomenon becomes aggravated as the electric power of the heater becomes larger, because the current fluctuation at on/off operation becomes even larger.
Also the flickering phenomenon becomes most serious when the electric power consumption of the fixing apparatus becomes smaller. For example, when the temperature control is executed from a cooled state of the fixing apparatus, all the waves are turned on because a large electric power is required, so that the consumption of the electric power from the power source scarcely fluctuates. On the other hand, when the fixing apparatus is warmed up and requires a low electric power, the electric power consumption shows a large fluctuation when the on-time (wave numbers) decreases with respect to the off-time, thereby aggravating the flickering phenomenon.
In order to avoid such situation, Japanese Patent Application Laid-open Nos. 09-258598 and 2004-138839 propose a method of providing the heating unit of the fixing apparatus with plural heating members and selecting a number of energized heating members according to a control state of the heating members, thereby switching an apparent resistance. In this method, more specifically, two heating members are simultaneously energized when a large electric power is required as in a start-up state from a low temperature, and one heating member only is frequently turned on and off in a state where the temperature reaches a predetermined value and is maintained constant. Such control method enables a high-speed temperature elevation from a low temperature state by simultaneously energizing plural heating members, and, during a temperature maintaining state, allows to reduce a fluctuation in the electric power consumption in on/off operations, thereby suppressing the flickering phenomenon.
As the electric power supply to the heating unit is usually executed directly from the commercial AC power source, it is necessary to have a same duty ratio in the positive and negative sides of the AC power supply in a unit time (positive-negative symmetry), in order not to give a detrimental effect on the power source. However, in a wave number control by a unit of half-wave for the temperature control of the heating unit, such positive-negative symmetry cannot be attained when the plural heating members have different resistances, thereby leading to a flickering problem. In case the heating members 1 and 2 have different resistances, a summed current waveform of the heating members 1 and 2 does not become symmetrical even in case of a positive-negative symmetry in the AC input.
On the other hand, a control in the full-wave unit can always achieve the positive-negative symmetry but leads to a rough control thereby inducing a temperature ripple. Also the flickering may increase because the on/off interval becomes longer.